In the meantime, I found a real (battery saving) brightness control method for my HP laptop (HP 250 G6 with Intel HD 520 graphics). It directly controls the backlight instead of the gamma values. The solution is based on https://wiki.archlinux.org/index.php/backlight

Although this draft is enough for Puli which allows machine-specific configuration for different machines on the same pendrive, I am planning to update this to a bit more complex pet where many graphic cards are handled, and the script utilizes xrandr only for the rest of cards.

Notice that even different Intel cards have different max_brightness values, e.g., 1500 in my G6 but 234 in my son's HP 250 G5 with Intel HD 5500 graphics.

For all (tested) Intel graphics, it directly controls the backlight using the Intel method. For the rest, as referenced a couple of posts above, it uses xrandr to control the gamma based on the nice method of Mike Walsh,

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